This invention relates to a method of synthesizing linear acrylic polymers and copolymers having controlled molecular weight based on photo-initiated free radical polymerization of vinyl monomers in the presence of chain transfer agents.
Synthesis of vinyl polymers by free radical initiated polymerization is known in the art. The ability to assemble linear vinyl polymers is complicated by several reaction pathways that are available to the initiation and propagating species. Radical polymerization of vinyl monomers is particularly sensitive to these reactions and are discussed in detail in "Radical Chain Polymerization" in Principles of Polymerization, 2nd Ed., G. Odian, John Wiley & Sons, Inc. (1981). Under certain circumstances, particularly at elevated temperatures, these side reactions can be used very effectively to control vinyl polymer molecular weight (e.g., chain transfer and chain terminators). However, these same processes are the source of defects which alter the properties of the polymer. Their impact on polymer properties is amplified as the molecular weight of the polymer decreases. Therefore, control of these side reaction is desirable. T. Corner in "Free Radical Polymerization: The Synthesis of Graft Copolymers", Adv. Polym. Sci., 1984, C2. 95, discusses how one can produce polymers with fewer defects. Low reaction temperature is a predominant factor in reducing the unwanted chain transfer and chain termination processes.
Thermal-initiated polymerization require initiators which decompose upon heating. There is a lower temperature limit for any thermal initiator below which it becomes ineffective in initiating polymerization. Very low temperature thermal initiators are not practical because of safety concerns relating to their instability under polymerization conditions.
Redox initiators are used at low temperature to polymerize vinyl monomers. Ionic redox initiator are ineffective for organic solution processes due to solubility issues. The organic soluble redox initiators contain chemical moieties which cause yellowing of the polymer or make the polymer more susceptible to oxidation, e.g., amines and metal accelerators. It is therefore desirable to conduct vinyl polymerizations at temperatures lower than conventionally achievable by these techniques.
Photoinitiated vinyl polymerizations work well at temperatures which are impractical for thermal initiators. The photoinitiators have the organic compatibility needed for effectiveness in solution processes and contain moieties which do not alter the performance of the polymer as do the organic soluble redox systems. Photoinitiators which work effectively at low temperature also produce polymers with fewer defects by virtue of the depression of undesirable transfer and termination reactions.
Control of molecular weight in vinyl polymerizations can be achieved with the minimum of undesirable side reactions by use of specifically designed chain transfer agents (CTAs). In free radical polymerizations, compounds which contain a sulfur-hydrogen (commonly known as a thiol moiety) are good CTAs for moderating molecular weight. They control the polymer molecular weight by hydrogen atom abstraction from the mercaptan by the propagating radical center. See, "Radical Chain Polymerization" in Principles of Polymerization, 2nd Ed, G. Odian, John Wiley & Sons, Inc. (1981). The deficiencies of this class of CTAs are well known to one skilled in the art. Offensive odor and deleterious effects to weathering properties has resulted in a search for other classes of CTAs.
The use of addition-fragmentation agents to control molecular weight are known. These chain transfer agents are effective at controlling molecular weight of vinyl polymers but copolymerize with monomers thus being ineffective as CTAs. As pointed out in "Addition-Fragmentation Processes in Free Radical Polymerization", Colombani, et al., Prog. Polym. Sci., Vol. 21, 439, 1996, and references therein, the chain transfer reaction is favored at elevated temperatures whereas copolymerization is favored at low temperatures. It is then anticipated that this class of CTAs would not be effective in low temperature polymerizations.
It is therefore surprising to find that addition-fragmentation CTAs provide good molecular weight control in acrylic polymerizations initiated by irradiating a photoinitiator and that photoinitiated vinyl monomer polymerization can be employed at relatively low temperatures to synthesize linear polymers.